Historically, acute myocardial infarction (AMI) events have been diagnosed in a patient when at least two of the following three criteria are satisfied: (1) ischemic chest pain typically associated with AMI, (2) electrocardiogram (ECG) findings typically associated with AMI (e.g., abrupt and sustained ST segment deviation and later appearance of pathological Q-waves), and (3) raised concentrations of troponin or creatine kinase (CK) in serum. Angina is diagnosed as pain or chest discomfort of cardiac source, elicited by exertion or emotional stress and relieved by nitrates: dangerous unstable angina (UA) is associated with new, progressive, or crescendo severity of symptoms. In both acute myocardial infarction (AMI) and unstable angina (UA), it is important to document to pattern of changes associated with symptoms and exertion, and to stratify risk by assessing the ECG abnormalities occurring during exercise.
It has been difficult for prior medical devices, either implantable or external, to detect and assess these conditions with satisfactory levels of sensitivity and specificity using the above criteria. For example, in AMI, troponin levels are difficult to access through an insertable loop recorder, while angina assessment has not been performed in association with activity parameters. These difficulties in detection have led to the late recognition of increased ischemic risk, or of ischemia, angina, or AMI events by a patient in seeking assistance. In all cases, this leads to long delays in recognition by the patient, further delaying actions by medical personnel, including measures to reduce risk and to deliver therapies to the patient, such as fibrinolysis or percutaneous coronary intervention (PCI).